British patent specification 1,370,894 "Improvements in or Relating to Extrusion" published Oct. 16, 1974 and U.S. Pat. Nos. 3,765,216 and 4,564,347, both assigned to Babcock Wire Equipment Ltd. of Ashford, England, disclose an extrusion machine having a rotatable wheel formed with two identical circumferential grooves and tooling disposed immediately adjacent the periphery of the wheel and provided with channels extending from positions adjacent the grooves to a mixing chamber having a portal mandrel; i.e., a mandrel centered in a die orifice. When, for example, coiled rod metal stock is fed into the wheel grooves and thence into the mixing chamber, a tubular extrusion discharges axially of the die orifice. The metal stock is fluidized by friction-induced heat and pressure in the wheel grooves and stock from the two grooves blends together and emerges as a single integral extrusion.
The machinery and process described immediately above may be used to produce both solid and tubular extrusion and to apply the feed stock as a cladding or sheath to a cored material which is fed axially through the center of the mandrel.
Two-wheel machines are also known, using the fundamental principle of the single wheel machine but feeding stock into the die from essentially laterally opposite input ports; i.e., the two wheels operate in mirror-image fashion relative to a central die assembly. See for example, U.S. Pat. No. 4,217,852, Re. 32,385. Specifically, the wheels rotate in opposite directions around spaced parallel axes and the tooling is disposed between the two wheels and adjacent peripheral portions thereof. While greater mechanical input power is required to drive two wheels than to drive one, the two-wheel machine is capable of substantially greater production rates than the single wheel machine. Moreover, the two wheel machine is believed to fill the extrusion cavity more uniformly and produce a more homogeneous product.
For many applications, extruded tubing must exhibit a high quality surface finish, uniform density and uniform wall thickness; e.g., wall thickness tolerances may be on the order of 0.0015". To achieve these results it is essential to achieve stability in the spatial relationships between the various components of the aforementioned machine, the die and the die holder. Variations in the gap between the wheels and the feed passage components produce swings in power consumption and extrusion stock temperature and can seriously degrade surface finish and density. Under some circumstances, actual metal-to-metal contact between the die components and the wheels may occur.
Similarly, shifts between the die and the mandrel which defines the inner diameter of the tube produce variations in wall thickness and tubing strength. Where thin wall, small diameter tubing is to be used, for example, in refrigeration systems which require the tubing to be bent for forming variations in wall thickness can produce unacceptable results.
Stability in the spatial relationships between the components of the die assembly and the wheels of the machine is particularly difficult to achieve not only in view of the pressures involved, but also in view of the substantial temperature gradients an-.d substantial temperatures excursions which are produced by friction and pressure. These temperature excursions produce thermal growth of various structural components, the directions and magnitudes of which must be predicted and/or controlled or compensated with high accuracy.